A corpse resurrected and mutated by a strand of recombinant DNA. It's only purpose seems to be to kill, therefore spreading it's DNA, creating more Necromorphs. It is also possible for a large mass of corpses to be joined into one form over decades, forming a hive mind, which can command all necromorphs.
There has only been one documented case of this hive mind existing, it itself being destroyed by a C.E.C. employee by the name of Isaac Clarke, on the planet of Aegis 7.